U.S. patent number 7,982,836 [Application Number 11/482,474] was granted by the patent office on 2011-07-19 for liquid crystal display device.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Yoshio Koike, Takashi Sasabayashi, Arihiro Takeda.
United States Patent |
7,982,836 |
Takeda , et al. |
July 19, 2011 |
Liquid crystal display device
Abstract
A liquid crystal display device in which a pair of substrates
having electrodes face each other, and liquid crystal is sealed
between the substrates. The liquid crystal display device includes
a insulating layer that varies electric field orientations in a
pixel region when a voltage is applied between the pair of
substrates.
Inventors: |
Takeda; Arihiro (Kawasaki,
JP), Koike; Yoshio (Kawasaki, JP),
Sasabayashi; Takashi (Kawasaki, JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
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Family
ID: |
18490959 |
Appl.
No.: |
11/482,474 |
Filed: |
July 7, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060250561 A1 |
Nov 9, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09886636 |
Jun 21, 2001 |
7113240 |
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PCT/JP99/06737 |
Dec 1, 1999 |
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Foreign Application Priority Data
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Dec 24, 1998 [JP] |
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10-368092 |
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Current U.S.
Class: |
349/129;
349/138 |
Current CPC
Class: |
G02F
1/133707 (20130101); G02F 1/133345 (20130101); G02F
1/1393 (20130101) |
Current International
Class: |
G02F
1/1337 (20060101) |
Field of
Search: |
;349/129 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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636 917 |
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Feb 1995 |
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EP |
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53-048542 |
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May 1978 |
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JP |
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01-120528 |
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May 1989 |
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JP |
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5-45636 |
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Feb 1993 |
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JP |
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6-43461 |
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Feb 1994 |
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JP |
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6-130394 |
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May 1994 |
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JP |
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6-273781 |
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Sep 1994 |
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JP |
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7-43698 |
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Feb 1995 |
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JP |
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7-43719 |
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Feb 1995 |
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JP |
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07-225389 |
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Aug 1995 |
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JP |
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8-50281 |
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Feb 1996 |
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JP |
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8-179344 |
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Jul 1996 |
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JP |
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8-262444 |
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Oct 1996 |
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JP |
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10-96929 |
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Apr 1998 |
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JP |
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WO 98/57222 |
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Dec 1998 |
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WO |
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Other References
VA. Konovalov et al., "Multi-domain Vertically Aligned Mode," SID,
May 17-22, 1998, pp. 1127-1130. cited by other.
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Primary Examiner: Le; Uyen-Chau N
Assistant Examiner: Smith; Chad H
Attorney, Agent or Firm: Greer, Burns & Crain, Ltd.
Parent Case Text
This is divisional of application Ser. No. 09/886,636, filed Jun.
21, 2001 now U.S. Pat. No. 7,113,240, which is a continuation of
International Application No. PCT/JP99/06737 filed Dec. 1, 1999.
Claims
The invention claimed is:
1. A liquid crystal display device in which first and second
substrates carrying respective first and second electrodes thereon
face each other across a liquid crystal layer, said liquid crystal
layer being sealed between the first and second substrates, said
liquid crystal display device including an insulating layer
provided on said first substrate such that the insulating layer
varies electric field orientations in a pixel region when a voltage
is applied between the first and second substrates; a pair of
polarizers being disposed at respective outer sides of said first
and second substrates; said insulating layer comprising a plurality
of insulating patterns each having a dielectric constant different
from a dielectric constant of a surrounding area surrounding said
insulating pattern, each of said plurality of insulating patterns
occupying a limited area in said pixel region and changing the
direction of an electric field passing therethrough when an
electric field is applied to said pixel region; said plurality of
insulating patterns controlling an in-plane direction of liquid
crystal molecules in said liquid crystal layer when a voltage is
applied across said electrodes, first and second horizontal
alignment layers being formed on said first and second substrates
respectively such that said first horizontal alignment layer covers
said first electrode on said first substrate and said second
horizontal alignment layer covers said second electrode on said
second substrate, wherein said first horizontal alignment layer is
formed on the first substrate so as to cover the insulating layer,
and the liquid crystal is a nematic liquid crystal with a positive
dielectric constant, each of said plurality of insulating patterns
existing between said first horizontal alignment layer and said
first electrode, wherein the insulating layer comprises a portion
which covers at least half the pixel region, and extends
continuously across a plurality of pixels and at least one data bus
line at substantially same thickness.
2. The liquid crystal display device as claimed in claim 1, wherein
the horizontal alignment layers of the pair of substrates are
subjected to rubbing in predetermined directions.
3. The liquid crystal display device as claimed in claim 1, wherein
said plurality of insulating patterns are connected with each other
by an insulating film in said surrounding area, and wherein each of
said plurality of insulating patterns has a thickness different
from a thickness of said insulating layer in said surrounding
area.
4. The liquid crystal display device as claimed in claim 1, wherein
an electric resistance of the insulating layer is higher than an
electric resistance of the liquid crystal.
5. The liquid crystal display device as claimed in claim 1, wherein
the electrode of one of the substrates is formed by a metal
electrode and used as a reflecting plate.
6. The liquid crystal display device as claimed in claim 1, wherein
said plurality of insulating patterns are connected with each other
by an insulating film in said surrounding area, and wherein each of
the insulating patterns comprises a horizontal alignment layer that
has a thickness different from a thickness of said insulating layer
in said surrounding area.
7. The liquid crystal display device as claimed in claim 1, wherein
an impedance of the insulating layer is lower or higher than an
impedance of the liquid crystal.
8. The liquid crystal display device as claimed in claim 1, wherein
the first and second electrodes are arranged in the absence of
slits.
9. The liquid crystal display device as claimed in claim 1, wherein
the insulating layer comprises patterned structures that correspond
to pixels.
10. The liquid crystal display device as claimed in claim 1,
wherein the insulating layer is provided on the first
electrode.
11. The liquid crystal display device as claimed in claim 1,
wherein the first and second substrates are transparent.
12. A liquid crystal display device in which first and second
substrates carrying respective first and second electrodes thereon
face each other across a liquid crystal layer, said liquid crystal
layer being sealed between the first and second substrates, said
liquid crystal display device including an insulating layer
provided on said first substrate such that the insulating layer
varies electric field orientations in a pixel region when a voltage
is applied between the first and second substrates; a pair of
polarizers being disposed at respective outer sides of said first
and second substrates; said insulating layer comprising a plurality
of insulating patterns each having a dielectric constant different
from a dielectric constant of a surrounding area surrounding said
insulating pattern, each of said plurality of insulating patterns
occupying a limited area in said pixel region and changing the
direction of an electric field passing therethrough when an
electric field is applied to said pixel region; said plurality of
insulating patterns controlling an in-plane direction of liquid
crystal molecules in said liquid crystal layer when a voltage is
applied across said electrodes; first and second horizontal
alignment layers being formed on said first and second substrates
respectively such that said first horizontal alignment layer covers
said first electrode on said first substrate and said second
horizontal alignment layer covers said second electrode on said
second substrate, wherein said first horizontal alignment layer is
formed on the first substrate so as to cover the insulating layer,
and the liquid crystal is a nematic liquid crystal with a positive
dielectric constant, each of said plurality of insulating patterns
existing between said first horizontal alignment layer and said
first electrode, the second electrode on the second substrate is
made narrower than the insulating layer in a direction
substantially parallel to a surface of the second substrate facing
the first substrate, and the insulating layer comprises a portion
which covers at least half the pixel region.
13. The liquid crystal display device as claimed in claim 12,
wherein said plurality of insulating patterns are connected with
each other by an insulating film in said surrounding area, and
wherein each of said plurality of insulating patterns has a
thickness different from a thickness of said insulating layer in
said surrounding area.
14. The liquid crystal display device as claimed in claim 13,
wherein an electric resistance of the insulating layer is higher
than an electric resistance of the liquid crystal.
15. The liquid crystal display device as claimed in claim 13,
wherein the electrode of one of the substrates is formed by a metal
electrode and used as a reflecting plate.
16. The liquid crystal display device as claimed in claim 12,
wherein said plurality of insulating patterns are connected with
each other by an insulating film in said surrounding area, and
wherein each of the insulating patterns comprises a horizontal
alignment layer that has a thickness different from a thickness of
said insulating layer in said surrounding area.
17. The liquid crystal display device as claimed in claim 13,
wherein an impedance of the insulating layer is lower or higher
than an impedance of the liquid crystal.
18. The liquid crystal display device as claimed in claim 13,
wherein the electrodes are arranged in the absence of slits.
19. The liquid crystal display device as claimed in claim 13,
wherein the insulating layer comprises patterned structures that
correspond to pixels.
20. The liquid crystal display device as claimed in claim 13,
wherein the insulating layer is provided on the first
electrode.
21. The liquid crystal display device as claimed in claim 13,
wherein the substrates are transparent.
Description
TECHNICAL FIELD
The present invention generally relates to liquid crystal display
devices, and, more particularly, to a liquid crystal display device
that has liquid crystal sealed between a first transparent
substrate and a second transparent substrate.
BACKGROUND ART
In recent years, liquid crystal display devices have been widely
used because of their advantageous features, such as thinness,
lightness, low driving voltage, and low power consumption.
Particularly, active-matrix liquid crystal display devices having
an active element for each pixel, such as TFT-LCDs (Thin Film
Transistor Liquid Crystal Displays) have been becoming comparable
with CRTs in terms of display quality.
However, the use of LCDs has been limited due to a narrow viewing
angle. In order to eliminate this problem, various techniques have
been suggested. Among those techniques, there are many techniques
in which electrodes are patterned so as to control the inclinations
of liquid molecules in various directions by changing the field
distribution in cells. However, the electrode patterning techniques
cause problems described later herein. The present invention can be
applied to all of the electrode patterning techniques, and easily
solve those problems.
First, LCDs that are generally used for display devices will be
described. At present, the most commonly used LCDS are TN (Twisted
Nematic) LCDs of normally white mode. FIG. 1A shows the panel
structure of such a TN-LCD. In FIG. 1A, TN liquid crystal 12 is
sandwiched by glass substrates having alignment layers 10 and 11
having orientated directions deviated by 90 degrees from each
other. Accordingly, the liquid crystal in contact with the
alignment layers 10 and 11 are aligned in the orientated directions
of the alignment layers, and the other liquid crystal molecules are
orientated along the aligned liquid crystal molecules. As a result,
the molecules are twisted through 90 degrees. Further, the liquid
crystal and the alignment layers 10 and 11 are sandwiched by two
polarizing plates 13 and 14 that are situated in parallel with the
orientated directions of the upper and lower portions of the liquid
crystal, respectively. When light impinges on the panel having the
above structure, the light passing through the polarizing plate 13
turns into linearly polarized light and enters the liquid crystal
12. Along with the liquid crystal 12 twisted through 90 degrees,
the light is also twisted through 90 degrees while passing through
the lower polarizing plate 14. Here, the display is in a bright
state.
As shown in FIG. 1B, a voltage is applied between the alignment
layers 10 and 11, thereby straightening the liquid crystal
molecules and eliminating the twist. However, on the surfaces of
the alignment layers 10 and 11, the liquid crystal molecules remain
along the alignment layers 10 and 11 due to the strong orientation
force. In this situation, the liquid crystal 12 is almost
homeotropic with the linearly polarized light, and no rotation of
the polarizing direction occurs. Here, the display is in a dark
state. When the voltage is zero, the display returns to a bright
state due to the orientation force y on the alignment layers.
Next, an IPS (In-Plane Switching) method that realizes a wider
viewing angle will be described. For instance, Japanese Patent
No.53-48542 and Japanese Patent No. 1-120528 disclose methods in
which an electric field in parallel with the substrates is
generated in liquid crystal layers. As shown in FIG. 2A, according
to this method, a pair of striped electrodes 21 and 22 are formed
on a substrate 20 on one side, and the liquid crystal molecules
located between the slit-like electrodes 21 and 22 are driven by a
lateral electric field. The liquid crystal 23 is made of a material
having positive dielectric anisotropy. When no electric field is
applied, the liquid crystal molecules are homogeneously aligned in
parallel with the longitudinal direction of the striped electrodes
21 and 22, as shown in the plan view of FIG. 2B (i.e., the liquid
crystal molecules are homogeneously orientated at an angle of
approximately 15 degrees, so that the direction of the liquid
crystal molecules becomes uniform when a voltage is applied).
When a voltage is applied between the striped electrodes 21 and 22,
the directors of the liquid crystal molecules having dielectric
anisotropy are changed, as shown in FIGS. 3A and 3B. In such a
liquid crystal display device, polarizing plates 25 and 26 are
arranged on and under substrates 20 and 24, with the polarizing
axes or the absorbing axes crossing perpendicularly to each other.
One of the polarizing axes is located in parallel director
direction, so that a black display can be realized when no voltage
is applied and a white display can be realized when a voltage is
applied.
There have been dramatic improvements in the TN-type TFT-LCD
production techniques, and, in recent years, the TN-type TFT-LCD
production techniques excel CRTs in contrast ratio and color
reproducibility. However, the LCDs have a narrow viewing angle.
Particularly, a TN-type has only a very narrow viewing angle in the
vertical direction. Viewed from some other direction, the
brightness of the black state increases, making the image whitish.
Viewed from the other direction, the display becomes dark, and
gray-scale inversion occurs.
When a voltage is applied to TN liquid crystal cells, the liquid
crystal molecules are inclined in some degree. At this point, the
birefringence of the liquid crystal layer makes the cells have a
gray scale transmittance. However, this is the case only when the
liquid crystal panel is seen from the front, and the liquid crystal
panel looks different when it is seen from an oblique angle. In
FIG. 4, the appearance seen from the left is different from the
appearance seen from the right. For instance, the liquid crystal
has little birefringence effect on the light directed from the
lower left to the upper right in FIG. 4. Accordingly, when seen
from the right, the panel looks black, not gray. On the other hand,
with the light passing from the lower right to the upper left, the
birefringence effect becomes larger. As a result, the display looks
even whitish and closer to white.
To solve this problem, it is effective to employ the technique of
multidomain. According to this technique, a plurality of inclined
directions of the liquid crystal molecules exist in one pixel.
Because of this, the left half of the pixel in FIG. 5 exhibits a
large birefringence for the light passing from the lower left to
the upper right (white display), and the right half of the pixel
exhibits little birefringence (white display). In such a condition,
the display appears to be a gray scale image to a human eye, as
long as the division size is small enough. When seen from the left,
the display appears to be gray for the same reason. From seen from
the front, the display of course appears to be gray because the
inclined angles of the liquid crystal molecules are uniform on both
left and right sides. Thus, uniform gray scale image can be
obtained throughout a wider viewing angle range.
To achieve a multidomain structure, a mask rubbing process shown in
FIG. 6 can be used. When an alignment layer is rubbed with a
rubbing roller made of nylon or polyester, the liquid crystal
molecules have tendency to be orientated in the rubbing direction.
Taking advantage of this tendency, alignment layers 32 and 33 of
substrates 30 and 31 are subjected to rubbing in the right
direction by rubbing rollers 34 and 35, as shown in FIG. 6A. Next,
a half of each pixel is subjected to masking with resists 36 and
37, as shown in FIG. 6B. The alignment layers 32 and 33 are then
subjected to rubbing in the left direction, as shown in FIG. 6C.
The resists 36 and 37 are then removed, and the substrates 30 and
31 are attached to each other, with liquid crystals inside, thereby
completing a liquid crystal cell having the left and right
orientation directions.
With the mask rubbing method, however, there exist many problems.
These problems include the low productivity due to the complicated
process, the limitation on the number of divisions (at least four
divisions are necessary to satisfy all the conditions with respect
to contrast, color, and gray scale, but the maximum number of
divisions is 2 because of the complicated process), and poor
controllability in the rubbing process due to the masking process.
For these reasons, it has been very difficult to mass-produce the
multidomain panels by the mask rubbing process.
Other techniques to solve the above problems and to achieve a wider
viewing angle include a method in which an electric field
distortion is caused in a cell so as to control the alignments.
However, there are other problems in this case, such as the
difficulty in patterning electrodes, poor yield, and higher costs
due to a larger number of processes. Furthermore, if a minutely
striped electrode is formed from an ITO (indium tin oxide) layer, a
voltage drop is caused at the end portion of the electrode,
resulting in display unevenness.
In the IPS method, the liquid crystals are switched in the
horizontal direction. As mentioned before, when the liquid crystal
molecules are aligned with an inclined angle to the substrates, the
birefringence varies with the viewing angle direction. The
switching is carried out in the horizontal direction so as to
steady the birefringence and obtain excellent viewing angle
characteristics. However, this method also causes several problems.
First of all, the response is very slow, because the switching is
carried out with an electrode gap of about 10 .mu.m in the IPS
method, compared with the switching with an electrode gap of about
5 .mu.m in the conventional TN method. The response time can be
shortened by narrowing the electrode gap, but each two adjacent
electrodes needs to have a different electrical potential to apply
an electrical field. Otherwise, short-circuiting will occur between
the adjacent electrodes, resulting in a display with defects. To
avoid such a problem, each two adjacent electrodes are formed on
two different layers, but this simply increases the number of
manufacturing processes.
Also, since it is difficult to form minutely striped electrode with
ITO, striped metal electrodes are employed instead. However, this
causes a loss in the aperture ratio. If the pitch of the striped
electrode is narrowed to increase the response speed, the
proportion of the electrode to the total area becomes large,
resulting in lower transmittance. (In reality, the transmittance of
the IPS method is only two thirds of the transmittance of the TN
method. If the electrode pitch is halved and the density of the
striped electrode is doubled, the transmittance becomes only one
third of the transmittance of the TN method.) Because of the above
reasons, the similar display quality (in terms of brightness)
cannot be obtained, unless the brightness of the backlight is
tripled, for instance.
In reality, when quickly moving dynamic images are displayed,
blurring occurs in the images. Further, to increase the response
speed, a panel is subjected to rubbing not in the direction of the
electrode but in the direction deviated by for example about 15
degrees from the direction of the electrode. If the rubbing is
performed in the direction of the electrode, the rotational
direction of the liquid crystal molecules in the middle of the
electrodes is not stabilized as one direction, resulting in a
longer response time. Therefore, by inclining the rubbing direction
by 15 degrees, even after this process, the response time is twice
that of the TN type. Through this process, the viewing angle
characteristics are not perfectly symmetric, and some gray-level
inversion occurs around the rubbing direction.
To explain this situation, the coordinate system of a polar angle
.theta. shown in FIG. 7A and the azimuth angle .phi. shown in FIG.
7B are determined for the substrates 20 and 24, the electrodes 21
and 22, and the liquid crystal molecules 23 shown in FIGS. 7A and
7B. FIG. 8A shows the viewing angle characteristics of the panel,
in which the gray scales are divided into eight levels from the
white state to the black state, and the brightness variations are
analyzed by varying the polar angle and the azimuth angle. The
shadowed portions in the figure represent gray-level inversion,
which are caused in the two azimuths (the 45-degree ranges of 60 to
105 degrees and 240 to 285 degrees in the azimuth angle .phi.).
FIG. 8B shows the transmittance variations of the 8 gray-level
displays with respect to the polar angle .theta. at an azimuth
angle of 75 degrees, which causes inversion. The gray-level
inversion is caused due to a drop in the white brightness.
In the IPS method, the gray-level inversion is caused due to a drop
in the white brightness in the two azimuths, and the viewing angle
characteristics deteriorate. By switching in the lateral direction,
viewing angle characteristics equivalent to a multidomain panel,
but the transmittance, the response speed, the productivity and the
market price are all sacrificed. Particularly, a low response speed
is not suitable for displaying dynamic picture images.
DISCLOSURE THE INVENTION
The principal object of the present invention is to provide a
liquid crystal display device that exhibits improved viewing angle
characteristics, restricts gray-level inversion, and has a short
response time.
To achieve the above object, the present invention provides a
liquid crystal display device that has a pair of substrates having
electrodes facing each other, and has liquid crystal sealed between
the substrates. The device includes an insulating layer that varies
electric field orientations in a pixel region when a voltage is
applied between the pair of substrates. Accordingly, the liquid
crystal molecules become perpendicular to the lines of electric
force (in a case of negative liquid crystal) or in parallel with
the lines of electric force (in a case of positive liquid crystal).
Because of the varied orientations of the electric force, a
brightness variation becomes smaller when seen from any direction,
and the viewing angle characteristics are improved. Furthermore,
gray-level inversion can be restricted.
BRIEF DESCRIPTION OF THE DRAWINGS
The other objects, features, and advantages of the present
invention will become more apparent from the following description,
with reference to the accompanying drawings.
FIG. 1 shows the structure of a TN-type LCD panel;
FIG. 2 shows the structure of an IPS-type LCD panel;
FIG. 3 shows the structure of the IPS-type LCD panel;
FIG. 4 illustrates the viewing angle dependence with a conventional
device;
FIG. 5 illustrates a conventional multidomain structure and the
viewing angle dependence;
FIG. 6 illustrates a mask rubbing method;
FIG. 7 illustrates the definition of polar angle .theta. and
azimuth angle .phi.;
FIG. 8 illustrates gradation inversion;
FIG. 9 is a sectional view illustrating the principles of the
present invention;
FIG. 10 illustrates the movement of liquid crystal molecules at a
time of voltage application;
FIG. 11 shows a zigzag pattern of a transparent insulating
film;
FIG. 12 illustrates the viewing angle characteristics of a device
in accordance with the present invention;
FIG. 13 shows the viewing angle characteristics of a conventional
device (TN);
FIG. 14 shows the viewing angle characteristics of the device in
accordance with the present invention;
FIG. 15 is a sectional view of a first embodiment of a liquid
crystal display device in accordance with the present
invention;
FIG. 16 is a plan view of the first embodiment of the liquid
crystal display device in accordance with the present
invention;
FIG. 17 is a perspective view of transparent insulating films and
transparent electrodes;
FIG. 18 illustrates a situation when a voltage is applied between
the transparent electrodes;
FIG. 19 shows the relationship between the applied voltage and
light transmittance;
FIG. 20 shows a response time when the applied voltage is
changed;
FIG. 21 shows a microscopic pictures of a test cell when the
voltage is variable;
FIG. 22 shows a microscopic pictures of a different test cell when
the voltage is variable;
FIG. 23 shows a microscopic pictures of a yet another test cell
when the voltage is variable;
FIG. 24 illustrates a modification of the sectional structure of
the present invention;
FIG. 25 is a plan view showing a modification of the transparent
insulating film in accordance with the present invention related to
the modification shown in FIG. 24;
FIG. 26 is an exploded perspective view of a modification of the
present invention;
FIG. 27 shows the viewing angle characteristics of the present
invention; and
FIG. 28 shows the viewing angle characteristics of the prior
art.
PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION
The following is a description of embodiments of the present
invention, with reference to the accompanying drawings.
FIGS. 9A, 9B, and 9C are sectional views for illustrating the
principles of the present invention.
In FIG. 9A, an upper ITO electrode 40 faces a lower ITO electrode
42, with a space being maintained between the two electrodes 40 and
42, so that liquid crystal 44 is sealed between the two electrodes
40 and 42. A transparent insulating film 46 as a dielectric
insulating layer is formed on the lower ITO electrode 42. A
vertical alignment layer (not shown) is formed between the liquid
crystal 44 and the upper ITO electrode 40 and between the liquid
crystal 44 and the ITO electrode 42 (or the transparent insulating
film 46).
In the structure shown in FIG. 9A, one transparent insulating film
46 substantially covers most of one pixel. The broken lines
indicate the lines of electric force when a voltage is applied
between the ITO electrodes 40 and 42. Because of the insulating
film 46 (preferably transparent), the lines of electric force
incline in the direction perpendicular to the ITO electrode 40.
When no voltage is applied between the ITO electrodes 40 and 42,
the liquid crystal molecules 45 of the liquid crystal 44 are
orientated perpendicularly to the surface of the ITO electrode 40,
as shown in FIG. 10A. FIG. 10 shows a vertical alignment layer 50
on the side of the transparent insulating films 46. When a voltage
is applied between the ITO electrodes 40 and 42, the liquid crystal
molecules 45 which are not covered with the transparent insulating
films 46 start inclining along the inclination of the lines of
electric force, as shown in FIG. 10B. If the applied voltage rises,
the liquid crystal molecules 45 located at the transparent
insulating films 46 start inclining, as shown in FIG. 10C. The
liquid crystal molecules 45 then go through the stage shown in FIG.
10D. When the applied voltage becomes high enough, all the liquid
crystal molecules 45 are almost parallel with the surface of the
ITO electrode 40 while actually being orientated perpendicularly to
the lines of the electric force.
By forming the insulating (preferably transparent) films 46 that
vary the orientations of the electric field in a pixel region, the
liquid crystal molecules become perpendicular to the lines of
electric force created by the applied voltage. The orientations of
the electric field vary, and a plurality of gradient orientations
exist for the liquid crystal. As a result, the brightness variation
becomes smaller over a wide range of viewing angles, and the
viewing angle characteristics improve. Also, the occurrence of
gradation inversion can be restricted.
In FIG. 9B, the upper ITO electrode 40 faces the lower ITO
electrode 42, with a space being maintained, and the liquid crystal
44 is sealed therebetween. The transparent insulating films 46 are
formed on the lower ITO electrode 42, while transparent insulating
films 48 are formed on the upper ITO electrode 40. The transparent
insulating films 48 are formed in a staggered state with respect to
the transparent insulating films 46. An alignment layer (not shown)
is formed between the liquid crystal 44 and the ITO electrode 40
(or the transparent insulating films 48) and between the liquid
crystal 44 and the ITO electrode 42 (or the transparent insulating
films 46).
In FIG. 9B, each transparent insulating film 46 covers most of one
pixel. The broken lines indicate the lines of the electric force
created when a voltage is applied between the ITO electrodes 40 and
42. Because of the transparent insulating films 46 and 48, the
lines of electric force incline in the direction perpendicular to
the ITO electrode 40.
In this manner, the transparent insulating films 46 formed on one
substrate are located in a staggered state with respect to the
transparent insulating films 48 formed on the other substrate, so
that the orientations of the electric field can greatly vary in the
pixel region when a voltage is applied between substrates.
In FIG. 9C, the upper ITO electrode 40 faces a lower striped ITO
electrode 43, with a space being maintained, and the liquid crystal
44 is sealed between the upper ITO electrode 40 and the lower ITO
electrode 43. The insulating film 48 facing the striped ITO
electrode 43 is formed on the upper ITO electrode 40. A horizontal
alignment layer (not shown) is formed between the liquid crystal 44
and the ITO electrode 40 (or the insulating film 48) and between
the liquid crystal 44 and the striped ITO electrode 43.
In FIG. 9C, one single transparent insulating film 48 covers most
of one pixel. The broken lines in FIG. 9C indicate the lines of
electric force caused when a voltage is applied between the ITO
electrodes 40 and 43. Because of the insulating film 48, the lines
of electric force incline in the direction perpendicular to the ITO
electrode 40.
Since the transparent insulating film 48 is formed on only one of
the substrates while the other substrate is formed by the narrow
striped ITO electrode 43, the orientations of the electric field in
the pixel region can be greatly varied.
As shown in FIGS. 9A, 9B, and 9C, according to the technique of
patterning insulating structures such as the transparent insulating
films 46 and 48 to pixels, the insulating structures are not
restricted by the pitch of the pixels, and a pattern can be
constantly formed with the optimum width and pitch for the
orientation of the liquid crystal. Furthermore, the insulating
patterns do not need to be joined to one another, but can be
independent of one another, allowing more freedom in design.
In the structure shown in FIG. 9B, the liquid crystal 44 is made of
a negative nematic liquid crystal, such as MJ961213 (manufactured
by Merck & Co. Inc.), and the alignment layers are made of a
vertical alignment layer, such as JALS-684 (manufactured by JSR
Corporation). The width of the transparent insulating films 46 and
48 is 55 .mu.m, and the gap between the transparent insulating
films 46 and 48 is 5 .mu.m. In this case, no rubbing process is
performed on the alignment layers, still, excellent liquid crystal
orientation can be obtained, and no display unevenness occurs.
In the structure shown in FIG. 9B, the transparent insulating films
46 and 48 are striped, as shown in FIG. 11, and bent at 90 degrees
by every predetermined length in a zigzag pattern. The display
characteristics of such a case were measured, and the viewing angle
characteristics shown in FIG. 12 were obtained. The viewing angle
characteristics shown in FIG. 12 were far better than the viewing
angle characteristics of the conventional TN liquid crystal shown
in FIG. 13.
In this manner, when the striped insulating layers are
zigzag-patterned, the electric field orientations are varied with a
voltage applied between the substrates. Further, the viewing angle
characteristics are improved, and the gray level inversion is
limited.
Although the gap width of the pattern is 5 .mu.m and the pattern
width is 55 .mu.m, no display unevenness due to a voltage drop is
found, and an excellent image display is obtained. In FIGS. 12 and
13, CR indicates the value of contrast.
In the structure shown in FIG. 9C, the liquid crystal 44 is made of
a nematic liquid crystal material with a positive dielectric
constant, such as ZLI-4792 (manufactured by Merck Co. Inc.), and
the alignment layers are made of horizontal alignment layer, such
as AL-1054 (manufactured by JSR Corporation). The width of the
striped ITO electrode 43 is 5 .mu.m, the width of the transparent
insulating films 46 and 47 is 55 .mu.m, and the gap between the
transparent insulating films is 5 .mu.m. In this structure, a
rubbing process in the longitudinal direction of the striped ITO
electrode 43 was performed on the horizontal alignment layer to
obtain the viewing angle characteristics shown in FIG. 14.
In the above embodiment, the present invention is applied to a
transmission type. However, it should be understood that the
present invention may also be applied to a reflective display.
Either one of the electrodes 40 and 42 of any of the structures
shown in FIGS. 9A, 9B, and 9C can be made of metal, and the metal
electrode can be used as a reflecting plate, thereby obtaining a
reflection-type panel.
FIG. 15 is a sectional view of a first embodiment of a liquid
crystal display device of the present invention. FIG. 16 is a plan
view of the structure of the first embodiment. This embodiment
corresponds to the structure shown in FIG. 9C. In FIG. 15, RGB
color filters 62, 63, and 64 are formed separately from each other
by black matrixes on one surface of the glass substrate 60. A
transparent electrode (ITO electrode) 68 is formed on each of the
color filters 62, 63, and 64. Further, transparent insulating films
70, 71, and 72 are formed separately from each other on the
transparent electrode 68 on each color filter.
Meanwhile, the transparent electrodes (or metal electrodes) 76, 77,
and 78 are formed on one surface of the glass substrate 74, facing
the transparent insulating films 70, 71, and 72. Also, a data bus
line 79 is formed to face each corresponding one of the black
matrixes 66. Liquid crystal 80 is sealed between the glass
substrates 60 and 74.
A shown in FIG. 16, the gate of a TFT 82 is connected to a gate bus
line, the source of the TFT 82 is connected to the data bus line
79, and the drain of the TFT 82 is connected to the transparent
electrode 68. In this manner, the transparent insulating films 70,
71, and 72 are formed independently of each other, so as to
facilitate the injection process of liquid crystal when the liquid
crystal is charged between the facing glass substrates.
FIG. 17 is a perspective view of the transparent insulating films
70 and 71, and the transparent electrodes (or the metal electrodes)
76 and 77. In FIG. 17, the structure is shown upside-down, compared
with the structure shown in FIG. 15. However, the horizontal
alignment layers formed on the side of the transparent insulating
films 70 and 71 are subjected to rubbing in the direction of the
arrow in the figure, while the horizontal alignment layers formed
on the side of the transparent electrodes 76 and 77.
FIG. 18 illustrates a situation in which a voltage of 5 V is
applied between the transparent electrode 68 and the transparent
electrodes 76 and 77. In this figure, the narrow solid lines
indicate equipotential surface that represents the electric field
distribution. The circles indicate liquid crystal molecules, and
the circles with wedges indicate the inclination of the liquid
crystal molecules due to the electric field. The thick solid lines
indicate the light transmittance with the inclination of the liquid
crystal molecules.
FIG. 19 shows the relationship between the light transmittance and
the voltage applied between the transparent electrode 68 and the
transparent electrodes 76 and 77. Here, the horizontal alignment
layer is made of AL3506, and the liquid crystal is made of positive
nematic liquid crystal, ZLI-4792 (manufactured by Merck & Co.
Inc.).
FIG. 20 shows each response time [msec] in cases where the applied
voltage is changed from 0 V to 2 V, 0 V to 4 V, 0 V to 6 V, 0 V to
8 V, and 0 V to 10 V. In this figure, each circle indicates a
turn-on response time (a change from black to white), each triangle
indicates a turn-off response time (a change from white to black),
and each square indicates a total turn-on and turn-off response
time. Here, the longest total response time is 90 msec or shorter,
and the shortest total response time is approximately 50 msec. This
is an improvement, compared with the case of an IPS in which the
longest total response time is 100 msec or longer, and the quickest
total response time is 60 msec.
FIGS. 21A to 21D illustrate display conditions in cases where the
width of each of the transparent electrodes 76, 77, and 78 is 3
.mu.m, the gap among the transparent electrodes 76, 77, and 78 is 6
.mu.m, and the applied voltage among the transparent electrodes 76,
77, and 78 is changed from 3.0 V to 5.0 V to 8.0 V to 10.48 V.
FIGS. 22A to 22D illustrate display conditions in cases where the
width of each of the transparent electrodes 76, 77, and 78 is 5
.mu.m, the gap among the transparent electrodes 76, 77, and 78 is
10 .mu.m, and the applied voltage among the transparent electrodes
76, 77, and 78 is changed from 3.0 V to 5.0 V to 8.0 V to 10.48 V.
FIGS. 23A to 23D illustrate display conditions in cases where the
width of each of the transparent electrodes 76, 77, and 78 is 7.5
.mu.m, the gap among the transparent electrodes 76, 77, and 78 is
15 .mu.m, and the applied voltage among the transparent electrodes
76, 77, and 78 is changed from 3.0 V to 5.0 V to 8.0 V to 10.48
V.
The liquid crystal is generally driven by alternating current
waveform, but with an improvement in the response speed, it is
necessary to take into consideration the influence in 1 frame (in
which a direct current is supplied), i.e., the influence from the
direct current waveform. Accordingly, the driving characteristics
of the liquid crystal show both the alternating current
characteristics and the direct current characteristics. Therefore,
the conditions of both characteristics need to be satisfied.
In view of this, the transparent insulating films 46 and 48
employed to exert a desired influence on the driving
characteristics of the liquid crystal need to be set under
predetermined conditions. More specifically, the transparent
insulating films 46 and 48 need to be set in such a manner as to
lower the electric field either in terms of the alternating current
characteristics or the direct current characteristics. In terms of
the direct current characteristics, the specific resistance .rho.
needs to be high enough to exert an influence on the resistance of
the liquid crystal layer. In other words, to set a value equivalent
to or higher than the specific resistance of the liquid crystal
(for instance, the liquid crystal for TFT driving is 10.sup.12
.OMEGA.cm or higher), the specific resistance .rho. needs to be
10.sup.12 .OMEGA.cm or higher, and more preferably, 10.sup.13
.OMEGA.cm or higher.
In terms of the alternating current characteristics, the
transparent insulating films 46 and 48 need to have the capacity
values (which are determined from the dielectric constant .epsilon.
and the sectional area) equal to or lower than the capacity value
of the liquid crystal 44 (i.e., a value equal to or higher than one
tenth as an impedance).
Accordingly, the influence on the electric field distribution can
be obtained, as well as the effects in the shape formed by the
inclined surfaces of the transparent insulating films 46 and 48.
Thus, more stable and rigid alignments can be achieved. The liquid
crystal molecules that are aligned vertically in this situation
stably exist and act as separation walls for the domains generated
on both sides. When a higher voltage is applied, the liquid crystal
in the transparent insulating films also start inclining. However,
the domains formed on both sides of the transparent insulating
films 46 and 48 act as partition walls for the liquid crystal
molecules on the transparent insulating films 46 and 48, and the
liquid crystal in the center of the multidomain incline in the
direction substantially in parallel with the stripes of the
transparent insulating films 46 and 48, thereby obtaining a very
firm orientation. To realize such a situation, the capacity value
of the transparent insulating films 46 and 48 in the multidomain
needs to be equal to or smaller than 10 times the capacity value of
the liquid crystal 44. Accordingly, the material for the
transparent insulating films 46 and 48 should preferably have a
small dielectric constant .epsilon. and a large film thickness.
More specifically, the dielectric constant .epsilon. may be
approximately 3, and the film thickness of the insulating films may
be 0.1 .mu.m or larger. With an even smaller dielectric constant
.epsilon. and thicker insulating films, more preferable functions
and effects can be obtained. In this embodiment, a novolak resist
having the dielectric constant .epsilon. of 3 and having
protrusions of 1.5 .mu.m in film thickness were used for the
aligned multi domain creation. As a result of this observation, it
was found that a very stable alignment was achieved. With such
insulating films being employed for the insulating films 46 and 48
on both sides, further preferable functions and effects can be
obtained. Besides the novolak resist, an acrylic resist
(.epsilon.=3.2) was measured, and the obtained results were
substantially the same as in the case of the novolak resist.
Next, modifications of the sectional structures shown in FIGS. 9A
and 9B will be described. FIG. 24A shows a sectional structure
similar to the structure shown in FIG. 9A. Vertical alignment
layers 50 and 52 are shown in FIG. 24A. In a modification shown in
FIG. 24B, the transparent insulating films 46 made of SiN or the
like are uniformly formed on the ITO electrode 42. After that,
processes such as ultraviolet ray irradiation on a shadowed portion
47 that is a part of the transparent insulating film 46, thereby
making the dielectric constant of the shadowed portion 47 smaller
than the dielectric constant (.epsilon.=3) of the transparent
insulating films 46. By doing so, the lines of electric force at
the time of voltage application is bent in the same manner as in
FIG. 9A.
In the modification shown in FIG. 24C, the transparent insulating
films 46 having thick portions 46A and thin portions 46B are formed
on the ITO electrode 42, and the lines of electric force at the
time of voltage application is bent in the same manner as in FIG.
9A. In the modification shown in FIG. 24D, the transparent
insulating films 46 are not formed on the ITO electrode 42, but a
vertical alignment layer 52 having thick portions 52A and thin
portions 52B is formed directly on the ITO electrode 42. The lines
of electric force at the time of voltage application is bent in the
same manner as in FIG. 9A, in accordance with the locations of the
transparent insulating films 46 shown in FIG. 24A.
As shown in FIG. 25 by shadowed portions, two transparent
insulating films 46 are formed for each one pixel, extending in
parallel with the gate bus line 90. Further in FIG. 25, the gate of
each TFT 94 is connected to the gate bus line 90, the source of
each TFT 94 is connected to the data bus line 92, and the drain of
each TFT 94 is connected to the ITO electrode 40.
As shown in FIG. 26, the glass substrates 60 and 62 having the ITO
electrodes 40 and 42 and the vertical alignment layers 50 and 25
formed thereon face each other, with a predetermined space being
maintained by a spacer. The liquid crystal is then sealed between
the glass substrates 60 and 62. The polarizing plates 64 and 66 are
arranged in such a manner that the absorption axes cross each other
at an angle of 90 degrees.
Here, the vertical alignment layer 52 formed on a color filter is
produced by transfer-printing and baking a vertical orientation
material, JALS-204 (manufactured by Japan Synthetic Rubber, Co.,
Ltd.). The transparent insulating films 46 on the side of the TFTs
94 are produced by transfer-printing, exposing, developing and then
patterning a photosensitive polyimide material. The vertical
alignment layer 50 is produced by transfer-printing and baking a
vertical alignment material JALS-204 (manufactured by Japan
Synthetic Rubber, Co., Ltd.). The glass substrates 60 and 62 are
attached to each other, with a spacer having a diameter of 3.5
.mu.m being interposed therebetween. A negative liquid crystal
having a negative dielectric anisotropy is then sealed between the
glass substrates 60 and 62.
The angle regions having gray-level inversion in this case are
represented by the shadowed portions in FIG. 27. The angle regions
having gray-level inversion in the conventional TN-type liquid
crystal display device are represented by the shadowed portions in
FIG. 28. From FIGS. 27 and 28, it is apparent that the angle
regions having gray-level inversion are much smaller in this
embodiment.
Since the liquid crystal has the negative dielectric anisotropy, a
transparent film having optically negative uniaxial properties in
the normal film direction (which satisfies the equation
Nx.apprxeq.Ny>Nz, where Nx and Ny represent the refractive
factor in the direction of the in-plane of the film, and Nz
represents the refractive factor in the direction of the normal
direction of the film) is attached to either one of the glass
substrates 60 and 62, so as to compensate the optical state at the
time of non-voltage application (black display). Thus, the viewing
angle characteristics can be further improved. Also, an optically
biaxial phase difference film (Nx>Ny>Nz) or an optically
uniaxial phase difference film (Nx>Ny.apprxeq.Nz) having
positive uniaxial properties in the in-plane of the film may be
laminated.
In this manner, in a liquid crystal display device having liquid
crystal sealed therein, an insulating layer is formed in a region
that is narrower than a pixel region. When a voltage is applied
between the pair of substrates, the insulating layer varies the
orientations of the electric field in the pixel region. In this
structure, when a voltage is applied between the pair of
substrates, the liquid crystal molecules become perpendicular to
the lines of electric force (in a case of negative liquid crystal)
or are located in parallel with the lines of electric force (in a
case of positive liquid crystal). Accordingly, the orientations of
the electric field vary, and a plurality of inclined orientations
exist in the liquid crystal. As a result, a brightness variation
becomes smaller when seen from any direction, and the viewing angle
characteristics are improved. Thus, gray-level inversion can be
restricted.
Also, each insulating layer may have a dielectric constant that is
different from that of the surrounding area. Thus, the orientations
of the electric field in a pixel region can be varied when a
voltage is applied between the pair of substrates.
Further, each insulating layer may have a dielectric substance that
has a different thickness from the surrounding area. Thus, the
orientations of the electric field in a pixel region can be varied
when a voltage is applied between the pair of substrates.
Also, each of the substrates may be provided with an insulating
layer in such a manner that the insulating layer formed on one of
the substrates is in a staggered arrangement with the insulating
layer formed on the other one of the substrates. Thus, the
orientations of the electric field can be greatly varied when a
voltage is applied between the pair of substrates.
Furthermore, each of the substrates may be provided with a vertical
alignment layer, and the liquid crystal may be made of a nematic
liquid crystal material with a negative dielectric constant. In
this structure, when a voltage is applied between the pair of
substrates, the liquid crystal molecules become perpendicular to
the lines of electric force, and a plurality of inclined
orientations exist in the liquid crystal due to the variation of
the orientations of the electric field. Accordingly, a brightness
variation becomes smaller when seen from any direction, and the
viewing angle characteristics can be improved. Thus, gray-level
inversion can be restricted.
Also, an insulating layer may be formed on only one of the
substrates, and the electrodes of the other one of the substrates
is made narrower than the insulating layer. Thus, the orientations
of the electric field in a pixel region can be greatly varied.
Furthermore, each of the substrates may be provided with a
horizontal alignment layer, and the liquid crystal may be made of a
nematic liquid crystal material with a positive dielectric
constant. In this structure, when a voltage is applied between the
pair of substrates, the liquid crystal molecules are situated in
parallel with the lines of electric force, and a plurality of
inclined orientations exist in the liquid crystal due to the
variation of the orientations of the electric filed. Accordingly, a
brightness variation becomes smaller when seen from any direction,
and the viewing angle characteristics are improved. Thus,
gray-level inversion can be restricted.
Also, the electric resistivity .rho. of each insulating layer may
be made higher than the electric resistance of the liquid crystal.
Thus, a desired influence can be expected on the electric field
distribution of the liquid crystal layer in terms of the direct
current characteristics.
Furthermore, the electrode of one of the substrates may be made of
metal and serves as a reflecting plate. Thus, a reflection-type
display can be obtained.
Also, each insulting film may be a vertical alignment layer that
has a thickness different from the surrounding area. Thus, the
orientations of the electric field in a pixel region can be greatly
varied when a voltage is applied between the pair of
substrates.
Furthermore, the impedance of each insulating layer may be equal to
or higher than the impedance of the liquid crystal. Thus, a desired
influence can be exerted on the electric field distribution in the
liquid crystal layer in terms of the alternating current
characteristics.
Also, each striped insulating layer may be zigzag-patterned by 90
degrees by the predetermined length. Accordingly, the orientations
of the electric field can be further varied when a voltage is
applied between the pair of substrates, and the viewing angle
characteristics can be further improved. Thus, gray-level inversion
can be restricted.
Also, a plurality of insulating layers may be formed independently
of one another, thereby facilitating the injection process of the
liquid crystal between the substrates.
* * * * *